Geophysical prospecting is employed to determine the subsurface structure of the earth, and in one form seismic waves are generated in the earth so as to be reflected by the subsurface layers. These reflected waves are detected and recorded, and, when properly interpreted, provide certain indications concerning the possible location of natural resource deposits, such as mineral deposits, petroleum deposits and the like. The generated seismic waves are introduced into the earth by the application of forces of considerable magnitude, such forces being produced either by detonations of explosives, through the movement of very large weight masses, or by acoustic energy.
The production of seismic waves by the detonation of explosives has many inherent disadvantages. The explosion frequently renders the surrounding portion of the earth unsuitable for similar subsequent wave generation at the same location, there is little or no control over the frequencies of sound waves generated by explosives, the consequent reflections are of such varied frequencies that the results cannot easily be interpreted, large surface waves which disturb the recording equipment are produced, and a relatively small amount of the energy from the detonation is utilized to produce the seismic waves.
In systems in which seismic waves are generated by the movement of large masses, generally referred to as land vibroseis exploration, the energy source consists of three major components: (1) the generator, or vibrator assembly, (2) an electrohydraulic power supply and its controls, and (3) the vehicle or prime mover. A typical crew would consist of four such sources along with a recorder-geophone system and other support equipment. The base plate of the generator against the earth's surface must remain firmly coupled to the surface throughout the cycle to transmit undistorted energy to the earth, thus requiring an excess of vehicle weight to the base plate to provided sufficient coupling force to the ground. Thus, the force output is limited by vehicle weight. In addition, considerable noise is generated at the surface which interferes with the detection and recording of the seismic waves. There is great difficulty in transporting the necessary equipment between various location and the number of areas in which it can be used are thus limited and such sources are only usable on the surface of the earth and cannot be utilized below the surface.
Among publications known to me disclosing and suggesting various means and methods of producing a seismic wave are the following U.S. Pat. Nos. 2,281,751; 3,221,833; 3,282,371; 3,292,143; 3,346,066; 3,641,811; 3,718,205; 3,831,138; 3,876,917; 3,900,826; and 4,402,063. Additional such publications known to me include Seismic Energy Sources, 1968 Handbook, published by United Geophysical Corporation, and presented at the 38th Annual Meeting of the S.E.G. of Denver, Colo., October, 1968; Marine Vibroseis System, published by the Seismic Engineering Company, 1133 Empire Central, Dallas, Tex. 75247; Seismic Energy Generators, Vehicles Which Generate Seismic Waves by Movement of Large Masses, known to be available to the trade from Dresser Industries, Inc., Houston, Tex.; Geo Space, Houston, Tex.; Mertz Iron and Machine Works, Inc., Ponca City, Okla.; Failing, Inc., Enid, Okla.; Industrial Vehicles, Inc., of Tulsa, Okla.; Seismic Engineering Company, 1333 Empire Central, Dallas, Tex.; and the Belmont Dinoseis from Belmont Manufacturing Co., Broken Arrow, Okla.
U.S. Pat. No. to Cloud, 2,281,751, dislcoses filling a flexible bag with water which is lowered to the bottom of the bore hole and a separate anchoring mechanism is inserted to hold the bag down. Unless the bag is perfectly constrained by the hole walls and bottom and the anchor, the bag extrudes into some crack and breaks. Also, pressure is directed to the bottom and top of the well bore rather than only to the walls of the well bore.
U.S. Pat. No. to Malmberg, 3,221,833 utilizes a piston and cylinder arrangement and shoes to engage the walls of the well bore which may become cocked due to hole irregularities causing binding of the pistons and malfunctioning and sticking of the tool in the well bore.
U.S. Pat. No. to Miller 3,282,371, discloses an expander similar to the Malmberg construction as an anchor only. A second anchor is provided on the earth's surface and hydraulic pressure is applied to move the anchors to and away from one another.
U.S. Pat. No. to Baker 3,376,949 discloses a single pulse device, the frequency of which is determined by adjusting the size and perforations and the character of a resilient outer tube, which frequency cannot be varied during operation.
U.S. Pat. No. to Mertin 2,424,108, is similar to the device of Baker with the same problems. Mertin's land apparatus is not programmable and appears to depend upon the physical characteristics of a resilient ball to provide frequency of modulation.
U.S. Pat. No. 3,718,205 to Fair discloses a transducer utilizing the to and fro operation of a piston which is transmitted mechanically through a shaft to other pistons to displace hydraulic fluid and pressure to a metal bellows.
U.S. Pat. No. 4,042,063 to Waters illustrates a shear wave vibrator in which a shear wave is generated with an air cushion principle.
Other of the foregoing seismic energy sources illustrates the state of the art; however, none of them discloses or suggests a method of and apparatus for generating a seismic wave which includes a tubular expandable outer wall closed at each end by rigid, high-strength closure members to which two different pressures are applied, one for anchoring the generator to the wall and the second of greater intensity to provide the radiated waves which are directed only to the walls of the well bores.
It would be highly desirable to provide apparatus and methods of generating seismic energy, which are not affected by non-similarities, in which weathering effect is reduced, which are programmable, permitting precise control of wave form and synchronization of multiple units, in which no hold-down is required when used in a well bore but is coupled and anchored to the earth wall, which is efficient in that its output can be tuned to not crush or fracture the earth as with dynamite but produces seismic energy within the elastic range of the earth and a large portion of its energy is not wasted, such as in conventional vibroseis and dynamite, which is flexible in that its force output can be increased readily and easily, which is economical with respect to initial capital costs, maintenance, transportation and operation, which is portable and lightweight, transportable by helicopters or mounted on much smaller vehicles than conventional land vibroseis, which permits multiple sweeps from the same position in a bore hole, and which can be operated as a single pulse source thereby having the advantages of dynamite explosions without the disadvantages of dynamite, such as hole damage.
It would also be desirable to provide such apparatus and methods of generating seismic waves which can be used for a variety of purposes, for example, on land or in sea, for geophysical prospecting, civil uses and the like, and which can be readily adjusted to fit circumstances of use.